The present invention relates to the field of optically-coupled/isolated solid-state relay switching circuits for direct connection to conventional 120 volt a.c. power sources; in particular, where a single pole double throw (SPDT) function is required for applying power to either of two loads.
Electro-optical combinations for coupling a controlling signal to a solid-state power switching circuit while isolating the controlling source from power line voltages, in effect forming a SPST (single pole single throw) function, are well known. An example of a light-responsive switching circuit is disclosed in U.S. Pat. No. 3,676,685. Such light-responsive SPST switching circuits may be divided into two categories:
(a) those which apply power to a load only when the light-sensing element is illuminated, and
(b) those which apply power to a load only when the light-sensing element is dark.
There is an unsatisfied need for a single solid-state switching circuit capable of performing the function of these two categories (a) and (b) simultaneously, to provide a single pole double throw (SPDT) capability for routing power to one or other of two loads. An obvious method of achieving ths result is to employ two switching circuits, one each of categories (a) and (b) as described above, and apply a common controlling light source to both light-sensing elements. However, such a system carries unnecessary penalties of cost and complexity, as well as a reliability weakness due to the dependancy on two separate light-sensing elements.